The limitations with 2 sites with different versions of VR are:
- You can only use the features of the earlier version (eg. between 5.8 & 6.0 you can't use compression as it is only available if both sites are 6.0 or higher)
- You have to pay attention to compatibility of VR with vSphere & vCenter versions at the site where VR is installed. Check the VMware Product Interoperability Matrixes
(note: this only applies when VR is used standalone - if used with SRM, vCenters have to match (which usually means VR versions have to match as well).
Can you verify that you are using compatible versions and post the specific step where you are seeing the error, and the exact error? Logs would be helpful as well.